Cranial perforators are special purpose drills which are used to bore holes through the skull during cranial surgery. Such holes may be needed to vent fluids from the region surrounding the brain, to provide small passageways to the brain for the insertion and removal of instruments, or to position a cranial saw for subsequent used in removing a larger piece of the skull.
Regardless of the end use of the hole being made, it is critical that the cranial perforator stop its boring action before it encounters--and thereby damages--the delicate dura tissue surrounding the brain, or the brain itself. To this end, cranial perforators of the type disclosed in U.S. Pat. No. 4,600,006 issued July 15, 1986 to John W. Baker comprise concentric inner and outer drills and a support and drive assembly that rotatably drives the drills so long as the inner drill encounters a surface offering a predetermined loading, such as cranial bone. When the predetermined loading is no longer encountered, such as when the inner drill penetrates all the way through the cranial bone, a clutch assembly disengages the inner and outer drills from the support and drive assembly.
The clutch assembly comprises a trio of slots in the base of the outer drill that define a trio of lips or dogs. The trio of lips or dogs each comprise a side wall surface that terminates in a planar drive surface that is inclined at a 45 degree angle with respect to the aforementioned side wall surface. The trio of slots is sized to receive a correspondingly-sized trio of lugs provided on the base of the inner drill. Rotational drive is transmitted from the drive assembly to the inner and outer drills via the trio of lugs.
Each of the lugs comprises a cam surface that is inclined with respect to the rotational axis of the inner drill. This inclined cam surface is formed so that it extends perpendicularly to the outer surface of the inner drill (as measured at the point of intersection of the inclined cam surface with the outer wall) along the entire length of the inclined cam surface.
The lug cam surfaces of the inner drill are positioned to engage the planar drive surfaces on the lips of the outer drill. In use, so long as the inner drill encounters a predetermined resistance to penetration, the lugs of the inner drill remain engaged with the support and drive assembly and the inner drill will cause the outer drill to rotate with it as a unit with the support and drive assembly. However, as soon as the inner drill no longer encounters the predetermined resistance to penetration, the cam surfaces of the inner drill coact with the drive surfaces of the outer drill to force the inner drill forward relative to the outer drill and the support and drive assembly sufficiently for the lugs of the inner drill to be disengaged from the support and drive assembly, whereby the inner and outer drills will slip relative to the support and drive assembly as the support and drive assembly is rotated.
Although the cranial perforators of the type disclosed in the aforementioned U.S. Pat. No. 4,600,006 function satisfactorily, it is believed that the design of the lug cam surfaces may prevent the cranial perforators from functioning optimally. Specifically, by constructing the cam surfaces so that they extend perpendicularly to the outer surface of the inner drill along the entire length of the cam surfaces, the planar drive surfaces of the outer drill engage only a small fraction of the total width of the cam surfaces of the inner drill as the cam surfaces slide along the planar drive surfaces. As a result of this "point" contact between the cam surfaces of the inner drill and the drive surfaces of the outer drill, the desired camming action between the inner outer drills may be adversely affected. In addition, in practice it has been found that substantial effort is required to manufacture cam surfaces which extend perpendicularly to the outer surface of the inner drill member along the entire length of the cam surfaces.